A membrane with internal passages is disclosed in U.S. Pat. No. 5,635,039 (Cisar et al.). This membrane is a proton exchange membrane (PEM) that separates the anode and cathode of an electrochemical device and serves the dual purposes of conducting protons and electronically insulating the electrodes. In order to have good conductivity for protons, the PEM must be kept moist. This is necessary to hydrate the ion exchange sites, such as sulfonate sites, on the polymer to allow for proton transfer through the membrane. The internal passages allow the direct supply of water to the membrane instead of relying upon humidification of the reactants provided to the anode and/or cathode.
Cisar et al. further disclose that the internal passages in the membrane can be formed by pressing ionically conducting material around a plurality of removable elements at sufficient temperature and pressure to fuse the material into a single membrane. After the material is fused, the elements are removed from the membrane to leave a passage for fluid. The removable elements may take any shape or form so long as the passages provide a substantially uniform flow of fluid throughout the entire membrane. The preferred removable elements are substantially parallel wires or tubes, but may be elements which are later removed through dissolution.
Cisar et al. also discloses an alternative method for forming a membrane with internal passages that includes applying a recast film of ionically conducting material onto solid tubes or sheets of the same material. The recast film is preferably applied in multiple coats with drying time between each coat, followed by baking under nitrogen at 100 degrees Celsius in order to cure the recast material.
A membrane having internal passages allows water to be provided to the open ends of the passages along one edge of the membrane and delivered throughout the membrane. The water may even be circulated through the passages and exit the membrane at the open ends.
One exemplary method for producing bipolar membranes discloses that a cation-exchange membrane and an anion-exchange membrane can be laminated using a mixture of polyethyleneimine and epichlorohydrin to bond the membranes to each other by curing (Japanese Patent Publication No. 32-3962). A second method discloses the bonding of a cation-exchange membrane to an anion exchange membrane by using an adhesive having the properties of exchanging ions (Japanese Patent Publication No. 3403961). A third method discloses a pasty material comprising vinyl pyridine and an epoxy compound that is coated on the surface of a cation-exchange membrane, followed by exposure to radiation to obtain the bipolar membrane (Japanese Patent Publication No. 338-16633). A fourth method discloses a sulfonic acid polymeric electrolyte and an allylamine that are adhered to the surface of an anion-exchange membrane, followed by exposure to ionizing radiation (Japanese Patent Publication No. 51-4113). Yet another method discloses a process in which a polyethylene film is impregnated with styrene and divinylbenzene followed by polymerization to give a sheet-like material. The sheet-like material is then nipped between frames made of stainless steel, where one side thereof is sulfonated, and thereafter, the sheet is detached and the remaining side is chloromethylated followed by treatment for amination (U.S. Pat. No. 3,562,139). However, these bipolar membranes exhibit inherently poor current efficiency and high-power consumption. For example, the use of these bipolar membranes to split water requires application of a membrane potential (e.g. 2.5 V to 3.0 V, or higher) that is much higher than the theoretical water-splitting membrane potential (0.83 V).
Bipolar membranes have also been prepared by coating the mating surfaces of cation-and anion-exchange membranes with a solution comprising at least one kind of inorganic electrolyte, such as sodium tungstate, chromium nitrate, sodium metasilicate, and ruthenium trichloride. The mating surfaces are placed in contact and pressed to give the bipolar membrane having a low water splitting membrane potential. This bipolar membrane, compared with the bipolar membranes discussed above, is characteristic of a low water-splitting membrane potential. However, the water splitting potential of this membrane increases over a relatively short period of use due to the development of bubbles or blisters at the interface between the cation-exchange membrane and anion-exchange membrane. Partially or entirely separated membranes are rendered useless. Furthermore, this membrane still provides low current efficiency and is not satisfactory for use on an industrial scale.
Attempts have been made to increase the stability of such bipolar membranes, obtained by combining anion-exchange membranes with cation-exchange membranes, by applying an ion permeable adhesive between the two membranes. A polymerizable mixture of polyethyleneimine and epichlorohydrin (U.S. Pat. No. 2,829,095) or polyvinyl chloride and polyvinyl alcohol (Israel Journal of Chemistry, 9 (1971), 485) have been proposed as an adhesive. It has also been found that bipolar membranes suitable for electrodialysis may be obtained from an anion-exchange membrane, a cation-exchange membrane and an ion-permeable adhesive comprising an aqueous solution of a polyvinylamine.
Bipolar membranes are difficult to produce by conventional methods. For example, in a chemical treatment of the surface, the two layers must have uniform thickness and must be in contact with one another over the entire surface area in order to ensure the current flow. On the other hand, the layers must not penetrate one another since the membrane would then lose its bipolar selectivity. Although combining two monopolar membranes gives bipolar membranes possessing defined anionic and cationic layers, this method gives rise to difficulties at the contact surface. If the membranes are not completely in contact, the resistance increases. The same applies where the adhesive is not sufficiently conductive. Moreover, very undesirable tears or bubbles may form at the points of contact in bipolar membranes of the stated type under typical operating conditions.
Bipolar membranes which consist of two individual membranes and polyvinyl alcohol as an adhesive may be prepared by a method in which the cation-exchange films and the anion-exchange films are coated with a polyvinyl alcohol solution. The cation-exchange and anion-exchange films are laid one on top of the other and heated for about one hour at about 60.degree. C. The bipolar membrane is then dried and compressed for about 30 minutes at about 100.degree. C. Although the resulting bipolar membranes exhibit firm adhesion, their swellability in aqueous salt solutions is irreversibly restricted, and these bipolar membranes, which possess rectifying properties, are therefore unsuitable for electrodialysis.
Despite certain advances described above, the performance of bipolar membranes is still limited by the transport of water into the interfacial region. U.S. Pat. No. 4,851,100 proposes to increase water transport to the interfacial region by using a continuous layer of a cation-selective material that is sufficiently thin to reduce the distance the water must diffuse to reach the interfacial region. This bipolar membrane is made by affixing a thin castable cation exchange membrane to a defined base anion exchange membrane. While a bipolar membrane of this construction might provide some increase in the rate of water transport to the interfacial region, the water transport rate is still limited because it has to pass through an ion-selective layer.
Therefore, there is a need for an ionically conductive membrane and methods of making ionically conductive membranes having internal passages that provide for communication of fluids into the membrane. It would be desirable if the membranes would provide direct communication of fluids to the interfacial region, as well as stability and low electrical resistance. It would be further desirable if the methods of making the membranes were simple and reliable.